Unit 9 - Techniques of Analysis of Flavors Flashcards
T or F: The lay and scientific literature typically claims that humans can discriminate 10,000 odors, but this number has never been empirically validated
T
Humans can discriminate at least ___________ olfactory stimuli
1 trillion
We determined the resolution of the human sense of smell by testing the capacity of humans to discriminate __________________
odor mixtures with varying numbers of shared components
What is the challenge we face with flavor analysis?
Flavors represent 0.1% or less of the food components and since they are present un such small amounts we really do not want to destroy them
Flavor is an integrated response composed primarily of the sensations of ________ and _______
aroma and taste
A key step towards understanding what constitutes the flavor of any foodstuff is to _________________________________ which act, either independently or in combination, to produce a highly characteristic aroma response for that particular product.
establish the chemical nature of those volatile constituents
Relative concentrations of the different flavor impact compounds =
flavor profile
In order to study the chemical basis of a flavor, it is necessary first to ________________________ from the nonvolatile bulk of the food and then by the judicious use of appropriate techniques of physical and chemical analysis to obtain the maximum of qualitative and quantitative information on all compounds of possible sensory importance
isolate the volatile aroma constituents
Name the 3 principal steps in flavour analysis:
1) Isolation
2) Separation
3) Identification (chemical structure)
It must be stressed, in regard to identification procedures, that the most powerful and widely used methods available for characterizing flavor volatiles are mainly instrumental, in particular mass spectrometry linked to _____________________
gas chromatography
What is the main interference in the food flavor analysis industry:
water
T or F: Flavor analysis is considered a complex task
T
The total aroma volatiles normally constitute only a very ______ proportion of the weight of the food, ranging from a few hundred ppm for some strongly flavored fruits to less than 10 ppm for some vegetables
minor
To obtain a sample for study, the chemist must remove the volatile components from the food, free them from large amounts of interfering non-odorous volatiles such as ______ and finally ________ them to a form suitable for analysis
water
concentrate
T or F: the correct choice of isolation and concentration procedures is crucial. The methods chosen must not only be efficient enough to yield an adequate amount of sample for analysis, buy must also give an isolate of high sensory quality. In example, on re-dilution the aroma should closely resemble that of the foodstuff from which it was obtained
T
Food aroma isolates are extremely ______________ frequently containing several hundred individual components. While the volatiles of certain products may be dominated by one class of compound, such as ______ in fruits and __________ in blue cheeses , the range of functional moieties which may be represented in food volatiles is very large indeed
complex mixtures
esters
methyl ketones
T or F: The flavor chemist is of course interested primarily in those food volatiles which exhibit important sensory properties, especially those which function as either character impact or contributory flavor compounds
T
T or F: sometimes the concentration of flavor compounds is too low for instruments to detect them
T
Striking aspect of food aroma is the tremendous range of olfactory ________________ exhibited by simple food odorants
detection thresholds
T or F: Trace constituents in an aroma complex are sometimes of far greater sensory importance than other volatile compounds present in vastly larger amounts.
T
The extreme sensitivity of the ________ in the detection of certain powerful odorants may exceed by several orders of magnitude that of even the most sensitive instrumental detectors available
human nose
The task of analyzing a food aroma isolate is then primarily one of ___________ complicated mixtures of organic compounds into their individual components and determining the _____________ of these components
separating
chemical strucure
Very few odorants have molecular weights in excess of _____
250
Foods consist of a complex matrix comprising _______, _______, _______ and water, the relative amounts of which may vary widely for different products. The major portion of the odorous fraction, which is composed of a range of compounds of widely different _________, ________, and _________ will be distributed in a highly specific manner throughout the bulk of the food
carbohydrates
lipids
proteins
- solubilities
- polarities
- volatilities
At equilibrium, the vapor directly over the food contains much smaller amounts of aroma compounds, and the composition of the volatiles in the vapor phase (headspace) depends on the __________ of the compounds in their pure state at the temperature of the food and on the interaction of these compounds with the food matrix.
vapor pressures
The preparation of a sample of food aroma volatiles may be approached in two different ways :
1) Total volatile analysis
2) Headspace analysis
The investigator attempts to isolate and concentrate from the foodstuff the totality of volatile constituents which could possibly contribute to flavor. The preparation of such concentrates followed by their qualitative and quantitative analysis has formed the basis for most of our understanding of food flavor to date:
total volatile analysis
Isolation and examination of the volatile compounds present in the vapor phase above the food material:
headspace analysis
Name the 3 important parts of headspace analysis:
- headspace (above the food product)
- a polymer resin used to trap the headspace volatiles
- filtered air drawn in over the sample surface
Both procedures have advantages and disadvantages. The preparation of concentrates of ___________ is quite complex and time-consuming and it requires great care to avoid altering important components or introducing ___________. However, it normally provides sufficient sample for comprehensive chemical and sensory analysis. On the other hand, ______________ may be performed on a small quantity of foodstuff with very little sample preparation, and the minimum artifact formation. An additional advantage is that the headspace vapor contains the aroma components in the relative proportions which one actually smells (ie: it represents the effectual odor of the food product)
total volatile
artifacts
headpsace analysis
In order to obtain an essence or concentrate of the total volatile constituents, the general procedure is to start with:
a relatively large amount of the product, and by the most appropriate choice of distillation, extraction and concentration procedures to obtain an extract possessing high sensory quality, followed by separation and identification procedures using various techniques such as GC/MS
The primary aim of any study on fresh fruit, fruit juice or other heat sensitive material is to preserve the desirable fresh aroma by isolating the volatiles with the mildest __________ possible. While it may be poissble with suitably designed equipment to remove volatiles without damage by _____________ at atmospheric pressure, distillation of volatiles from these products is best performed at ___________ by operating under vacuum
treatment
flash evaporation
low temperatures
T or F: with vacuum distillation you need to start with a big amount of product to have a high concentration at the end
T
For products whose flavor is not damaged by heating, or where specific types of cooked flavors are the subject of an investigation, then direct steam ____________ at atmospheric pressure is frequently the method of choice
distillation
A particularly convenient and effective procedure is one which combines steam distillation of the food volatiles with simultaneous __________ of the volatiles into a small quantity if a low-boiling water immiscible ___________ such as dimethyl ether, also known as SDE ____________________
extraction
organic solvent
simultaneous distillation extraction technique
T or F: The CO2-acetone condenser minimizes the loss of lower-boiling volatiles
T
When did we start to see the use of GC in flavor research:
In the 1960s (direct analysis of the vapors in equilibrium with foodstuffs)
Direct sampling and cryogenic trapping = _____ volume
low
Direct injection of a small volume (____mL) of the vapor (collected by a syringe) over a food into a _____________ is the simplest form of headspace analysis and was used at an early date to monitor volatiles of ripening banana, and to follow the development of oxidative ________ and ________ in dehydrated potato granules. Several workers have achieved a 10-50 fold increase in the concentration of headspace volatiles over that obtainable in direct sampling by means of __________
5-10 gas chromatograph rancidity browning cryogenic trapping
T or F: we need to keep the cryogenic trap intact and cooled down until we analyze it because we do not want to dissolve the volatiles
T
T or F: Charcoal has a porous surface, is non-polar and can trap volatiles
T
The volatiles over the food are swept with a stream of ________________ such as helium and condensed in a suitable liquid-nitrogen cooled trap, from which they may be subsequently desorbed by heat onto a __________
purified carrier gas such as helium
GC column
Cryogenic trapping is normally limited to a few hundred mL of headspace vapor, due to the retention in the traps of unacceptably large amounts of ________, which may seriously hamper the subsequent analysis
water
The use of various desiccants as ___________ to selectively remove water during trapping has been investigated, but even the best of these ________________, lowered the recoveries of many oxygenated compounds. While the major volatile components of a food may be readily analyzed, it is frequently impossible to isolate higher boiling compounds of sensory importance in amounts sufficient for instruments measurement.
pre-columns
anhydrous potassium carbonate
The use of porous polymers provides us with a higher ______
volume
A very effective approach which has greatly expanded the power of headspace technique, and which is now becoming well established practice in many laboratories, is the trapping of large volumes of headspace vapor on ________________. The outstanding property of these materials is their ability to retain most __________________ very strongly at ambient temperatures while displaying little affinity for water
porous polymer adsorbents
volatile organic compounds
Two series of these polymers are commercially available, namely the _________ series and the ______________, the different members within each series possessing somewhat different ____________ and polarities
Porapak
Chromosorb Century series
surface areas
Conditioning temperatures are __________ than desorption temperatures
higher
Step 1: For headspace trapping a quantity of the polymer, usually in the range ________mg, is packed into a glass tube and _____________ before use by purging with highly purified _____________ (to prevent oxidative damage to the adsorbents) nitrogen at temperatures ranging from _______, depending on the polymer
100-500
conditioned
180-225C
Step 2: The polymer trap is then __________ to the sampling apparatus containing the foodstuff and the volatiles are swept into the trap using a stream of purified ____ or ____________. Flow rates, while commonly around 50 ml/min can ba as high as ______ml/min, and volatiles may be collected from headspace volumes ranging from as little as one liter up to 250 or more
attached
air
nitrogen
300
Step 3: After collection the trap is disconnected and purged with purified __________ to remove most of the water. Nitrogen is passed through the trap in the opposite direction to that in the ________ step
nitrogen
collection
Step 4: The trap is heated to a specified temperature (100-___C) and the volatiles are desorbed and collected
150
Normally the vapor over a foodstuff is perceived under conditions where the volatiles are in ___________ with the bulk of the food, their concentrations being determined primarily by their respective ________________ and their concentrations in the food (Henry’s Law). These conditions no longer apply when the volatiles are removed by a gas stream, the diffusion rates of the volatiles from the bulk of the food to its surface then becoming __________ limiting factors
equilibrium
vapor pressures
kinetically
Low concentration of aroma volatiles can be isolated and concentrated on porous polymer traps such as ________ using purge and trap apparatus. Subsequently the trapped volatiles can be thermally desorbed into a ____________ using thermal desorption devices mounted on gas chromatograph. Both solid and liquid samples can be analyzed, if proper sampling apparatus is used. In both cases, samples are purged with purified inert carrier gas for a specific period of time (20-90 min) at a flow rate in the range of 10-80ml /min. The temperature, which can range from sub-ambient to few hundred degrees centigrade, depends on the nature of the sample. The adsorbent trap is then removed and fitted with a syringe needle and attached to a thermal desorption device.
Tenax-GC
GC-column
What is the role of the cryo trap in the GC column:
improve peak separation and detectability which is why we cool the column
The traps are thermally desorbed for a period of ________ minutes at temperatures depending on the nature of the sample (30-350C)
3-10
During the desorption period, the temperature of the GC column is maintained at ___________ values to cryofocus the aroma volatiles. After the desorption phase, the volatiles can be separated and eventually identified, by initiating the temperature programming of the column
sub-ambient
What are the 4 steps involved in solid-phase extraction:
1) Conditioning
2) Retention
3) Rinse
4) Elution
Solid phase microextraction (SPME) is a simple _______________technique that eliminates some of the problem associated with solvent extraction of aroma volatiles
adsorption/desorption
Name some systems used to absorb aroma compounds from samples for analytical purposes :
- traps loaded with various adsorbents
- spe (disk in a holder assembly)
- spme (coated needle inserted in a sample)
- twister
Polydimethylsiloxane:ideal for many ______ analytes, especially amines
polar
Carboxen: ideal for gaseous/__________ analytes, high retention for trace analysis
volatile
Carbowax: For ______ analytes especially for alcohols
polar
The device used to perform solid phase micro extraction consists of a one cm _____________, coated with a specific stationary phase (polydimethysiloxane, carboxene, etc.) and bonded to a stainless steel plunger and encased in a holder. The fused silica fiber can be drawn into a ______________ attached to the holder by using the plunger. The device can be used to adsorb aroma compounds either from a solution or from the ______________ above the solution, by inserting the needle through the septum that seals the sample vial. After sample adsorption, the needle can be introduced into the gas chromatograph injector where the adsorbed volatiles are thermally desorbed into the GC column and analyzed. The adsorption equilibrium is usually attained in 2-30 minutes depending on the concentration of the sample and the thickness and the type of fiber coating
fused silica fiber
hollow needle
headspace
What is GCO:
Gas Chromatography-Olfactometry
Although gas chromatography is used mainly in the quantitative analysis of flavor mixtures, however, when the instrumental detector is replaced with a human nose - through the use of a ________________ - GC can also be used to perform ______________
sniffing port
sensory analysis
The main application of GCO is to identify the odor-active components and _______________ of individual components present in a mixture and separated on a GC column,
odor quality
The most common techniques that utilize GCO include _____________ and ______________. Both techniques are based on dilution analysis and produce quantitative estimates of relative potency of compounds eluting from a GC column. In dilution analysis, the aroma extracts are _______________ and each dilution is analyzed until no significant odor could be detected. In AEDA, the number of dilutions required to eliminate the odor is used to estimate the potency of that particular compound.
CharmAnalysis
Aroma Extract Dilution Analysis (AEDA)
serially diluted
In CharmAnalysis, ______________ are included in the calculation of the potency factors which are known as charm values
retention times
Name the 4 main parts of a GC/MS system:
1) Gas (He, N2, H2)
2) Sample injector
3) T regulated oven with a packed or capillary column
4) Mass spectrometer detector
Serial dilution - Gas Chromatography - _____________ - Data Analysis - Graphics
Sniffing
What are sets of sensory arrays and we use them for what?
- Polymers sensitive to volatile compounds
- They are used in electronic noses
array = more than ___ detector
1
T or F: electric noses give you the fingerprint of a particular food product, often used for quality control in companies (make sure the aroma is consistent during production)
T
The term electronic nose refers to the use of _______________ to detect odors, where each sensor has only _______ specificity to a wide range of odorant molecules; coupled with a suitable pattern recognition system (software). Each chemical sensor in an electronic nose, represents a group of _______________ and produces a _______________________ in response to an odor. The final destination in the human olfaction process is the _________________, which classifies and memorizes odor; the equivalent process in the electronic nose is the use of ______________________
- multi-sensor arrays
- partial
- olfactory receptors
- time-dependent electrical signal
- cerebral cortex of the brain
- pattern recognition software
Many types of materials have been developed to be used as sensory arrays. These include ____________ and conducting polymers.
metal oxides
Metal oxides contain _____________ oxygen species with which interaction of the odor molecules alters the _____________ of the oxide. EN using metal oxides as sensors operate at ________________
chemisorbed
conductivity
elevated temperatures (400-600C)
The unique electrical properties of conducting polymers (work at room temp) such as __________________ and ________________ have been exploited also, since these polymers change their conductivity rapidly and ____________ in the presence of odors. The absorbed molecules are believed to cause a swelling of the polymer and to interfere with the _________________ within the polymer. These sensors can be used at room temperature and have a good _____________
polypyrroles polyanilines reversibly charge transfer sensitivity (0.1-100 ppm range)
T or F: sensory arrays have different structures and will react / detect different aroma chemicals
T
T or F: GC used by itself without a detector is useless
T
GC/MS requires a very ____________ amount of sample and very little time is needed for analysis
small
MS detector is one of the only detector that gives us complete ___________________
structural information
Although the chemicals actually responsible for flavors are generally low molecular weight (_____________________) volatile compounds, many substances used in flavors are made up primarily of __________________________. In order to detect these products when they are used in flavor, it is necessary to analyze both ____________ and ___________ that make up the bulk of the product.
less 400 amu
non-volatile compounds
volatiles non-volatiles
Volatile components are usually identified by the combined _______________________technique which remains the most powerful technique available to the chemist for the separation and identification of complex mixtures, it can generate the maximum amount of __________________ for the smallest amount of sample in the shortest time.
gas chromatogaphy/mass spectrometry
structural information
What is important to keep in mind for the interface between the GC and the MS:
We need to keep equal or higher than the one in the column because if the volatiles are too cold they will condense and they won’t enter the mass spectrometer.
T or F: Longer columns tend to be better at separating compounds
T
The analysis of non-volatile components such as __________, _________, _________, __________, etc is also important since the taste sensations (sweet, sour, bitter and salty) are produced primarily by non-volatile compounds. As in the case of the volatiles, the first task is to separate the complex mixture into ______________ which can be identified by spectroscopy. Separation of non-volatile components can be achieved by __________________, using reverse phase, normal phase or ionic phase columns in any of their modifications.
suagrs hydroxy acids amino acids flavanoids pure components high-performance liquid chromatography
There is no general methodology that will identify in a single analysis any major non-volatile components as GC/MS does for the volatiles for several reasons: First LC columns do not have the ________________ of GC column (10,000 theoretical plates vs 250,000 plates for capillary GC columns). Second, the carrier in LC is liquid hence it is difficult to remove it in order to ____________ the LC with MS for identification of the peaks. Unlike GC, current LC technology necessitates extensive method development for each class of compounds to be analyzed.
separation power
interface
T or F: LC is more limited because we have to purchase different columns for different types of compounds
T
LC path - enter at room temperature - separate with no chemical changes using rapid and mild method - __________________ - MS (ESI mode) can analyze non-volatile liquids
interface convert liquid into spray
GC path - enter at 250C - heat changes chemicals and creates new chemicals - MS (EI mode) can analyze only ____________________
volatile compounds
A principle concern of the flavor chemist must be the choice of the ___________________ which will be used for a particular separation and the ________________ which will allow optimal column performance. Flavor and aroma materials are commonly complex, thus dictating high resolution gas chromatographic separation prior to mass spectrometric analysis.
type of column
operational parameters
Generally speaking, the considerations for successful operation of a GC/MS system are the same as those for operation of a gas chromatograph alone, except that more attention must be paid to _________________. Among factors for consideration are _____________ and the __________________ used in it , gas flow rate, temperature programming and sample load .
- optimization of the conditions
- type of column
- nature of the stationary phase
What are the 2 types of chromatographic columns:
- capillary
- packed
Packed columns are ________ in diameter and require relatively _______ flow rates of carrier gas. Although capable of accomodating large sized samples, the ability of the mass spectrometer to handle the effluent (and the high carrier gas flow) is a limitation; hence the development of _________ and _______________ of various types which reduce the total volume of gaseous material passing to the _______________ of the mass spectrometer; in the case of separators, sample concentration is increased relative to carrier gas. Capillary columns, while of ____________ bore, are usually of greater length which results in greater _____________
larger high splitters separators ion source smaller resolution
There are literally hundreds of stationary phases. Categorization is based primarily on the ___________ and thermal ___________ The quantity that most concerns the researchers however, is not the total amount of component but that rate at which it enters the spectrometer
polarity
stability
Chromatographic operating conditions must be selected accordingly to achieve column performance at highest ___________.
efficiency
Where high boiling organic liquids are used as the stationary phase of the chromatography column, there is always an ________________ above which the liquid phase may decompose or volatilize sufficiently to produce a background of eluent that is commonly known as ___________________
upper temperature limit
column bleed
Many of the compounds found in the column bleed are _________________ polymeric substances whose spectra contain many fragment ions resulting in a high background spectrum in the spectrometer. Careful consideration must be given to the choice of column _______________ to minimize column bleed, if it cannot be avoided completely, to the choice of a substance whose mass spectrum contributes the least amount of background interference. Other background interference in the spectra of GC eluents may be due to ________ in the transfer tubes between the column and the spectrometer ion source. If low volatility components eluting from the column condense on the _____________, they may volatilize later at a slow rate, producing an effect to column bleed.
high molecular weight
liquid phase
cold spots
cold surface
Care must also be taken to avoid artifacts due to condensation in the GC inlet if the temperature is __________, or due to decomposition of the sample components if the inlet is _______. The efficiency of programmed temperature gas chromatography to afford ____________ separation of complex mixtures is well established, but in GC/MS some features, of little significance
too low
too hot
increased
Low resolution =
molecular weight
High resolution =
elemental composition
Responsible for fragmentation (break down the molecules into characteristic fragments):
ion source
Separates the smaller fragments based on their mass to charge ratio:
mass analyzer
Mass spectrometer only detects ________ species
ionic
What are the 2 types of mass spectrometers available:
low resolution and high resolution
T or F: Low resolution instruments are capable of distinguishing only between ions whose mass to charge ratios (m/z) differ by _______________. High resolution instruments are capable of distinguishing between ions whose masses differ in the ______ decimal place or less
one mass unit
third
What are the 3 major functional parts to all mass spectrometers:
1) the ion source (to produce ions from the molecules under investigation)
2) the mass analyzer (to separate these ions according to their mass-to-charge ratio (m/z)
3) the detector (to detect and measure the relative abundance of each ion)
T or F: mass analyzer operates under vacuum
T
+. =
radical cation
A mass spectrometer bombards organic molecules in gaseous phase with a beam of high energy particles such as ________________ causing them to ionize and fragment - if the particles are electrons the technique is called _____________________
electrons/atoms/molecules
electron ionization mass spectrometry
T or F: The extent of fragmentation of the molecule is proportional to the energy of the electron beam and the structure of the molecule
T
Normally, the energy of the electron beam is ______ in order to obtain reproducible fragmentation patterns
70 eV
The identification of ______________ thus allows ready determination of the molecular weight of the sample
molecular ions
Molecular ion =
radical cation
These excited ions therefore tend to get rid of this surplus energy and to stabilize themselves by ______________. This process is known as fragmentation.
bond cleavages
T or F: The way a molecular ion fragments can give us highly useful information about the structure of a complex molecule
T
T or F: Positively charged ions when they encounter a magnetic field will then follow a curvature
T
What is the simplest mass analyzer available:
a magnet
1/2 mv2 = zV
kinetic energy = energy supplied by the accelerating plates
r = mv/___
zH
m/z =
H2R2/2V
In a mass spectrum, the most intense peak is called the _____________
base peak
T or F: The base peak is always the peak arising from the molecular ion
F
The separation of the complex mixture into its components is achieved on the _____________________, the separated components are then introduced through the GC/MS interface into the ionization chamber of the mass spectrometer, where the molecules are bombarded with a beam of energetic electrons (electron ionization mode) causing them to ___________ and __________ in a way which is characteristic of the molecule
gas chromatographic column
ionize and fragment
T or F: In most cases where the spectra of authentic reference compounds are not available, the spectra nevertheless provides information about the molecular weight and features of structure that often permit the identity of the components to be deducted
T
